548 SCIENCE PROGRESS 



The latter are regarded as translated parallel to the C-axis in 

 such a way that each oxygen is approximately in the centre 

 of a tetrahedron formed by four zinc atoms. 



One of the most interesting investigations of crystal 

 structure ever carried out is that described by G. Bartlett and 

 I. Langmuir {Jour. Amer. Chem. Soc, 43, 84, 192 1). The work 

 of W. H. and W. L. Bragg {X-Rays and Crystal Structure, 

 London, 158, 1916) on the alkali halides showed that, in the 

 case of the potassium and sodium salts, each metal atom was 

 equidistant from six halogen atoms, and, correspondingly, each 

 halogen equidistant from six atoms of the metal. So far as 

 ammonium chloride is concerned, each ammonium radicle is 

 symmetrically placed with reference to eight halogen atoms, 

 and conversely. The structure in the latter instance agrees, 

 as a whole, with the centred cube, compared with the simple 

 cube of the former. This has been interpreted as indicating 

 a tetrahedral symmetry for the ammonium ion, while the 

 symmetry of the alkali ions is cubical. In order to test this 

 interpretation, Bartlett and Langmuir, by means of the method 

 devised by Hull [Phys. Rev., 10, 661, 191 7 ; Jour. Amer. Chem. 

 Soc, 41, 1 168, 1919), have examined the structures of the 

 ammonium halides, both at ordinary and higher temperatures. 

 Ammonium chloride and bromide each exist in two poly- 

 morphous modifications, the transition temperatures being 

 184-3'' ^nd 137-8° respectively, and hence the crystal structures 

 determined at 20° and 250° might be expected to be different. 

 This has proved to be the case, as in each salt the high- 

 temperature form has a simple cubic arrangement of the ions 

 analogous to that of sodium chloride, while the low-temperature 

 form has the more complex centred-cubic form. The transition 

 temperature for ammonium iodide is — 17-6°, so that the form 

 stable at 20° corresponds to the high-temperature form of the 

 chloride and bromide, and has a simple cubic structure. 



In the high-temperature forms, owing to the greater distance 

 between like ions, the influence of the ammonium ion becomes 

 less, and hence there is a greater tenden^ for the ammonium 

 salts to be isomorphous with the corresponding salts of the 

 alkale metals. A similar isomorphism can also occur where 

 the anion has a comparatively large volume, as in certain 

 sulphates, the influence of the ammonium ion being negatived 

 by that of the anion. 



The structure of caesium di-chloroiodide has been investi- 

 gated by R. W. G. Wyckoff {Jour. Amer. Chem. Soc, 42, iioo, 

 1920). Although two polymorphic forms — hexagonal and 

 rhombic — of this salt have been described (Wells and Penfield, 

 Amer. Jour. Sci. (3), 43, 29, 1892), there is apparently only one 

 form, the " rhombic " nature of one modification being due to 



